You can buy the OEM dust plates ($44 each) and rivet in aluminium flanges ($8-10, from an aircraft supplier). I did this and f'ed up by putting the hole so far from the strut it was over one of the wishbone thingies. Epic fail. You can make holes in the front and rear of the undertray side elements with a circular bit and rivet in a flange at the front. You can connect these with aircraft ducting ($8-10/ft, need ~8ft, $64-80).

(I have some 3" duct tubing for sale if you want! I was about to restart the dust plate cutting when another board member advertised a Cantrell set, so I decide to pick that up. The 3" tubing was also going to rub a fair bit in the wheel well, so I would have needed to replace all the above items to finish my DIY project.)